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tankless system advice
Last Post 25 Aug 2010 06:07 PM by Dana1. 33 Replies. |
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BadgerBoilerMN
Veteran Member
Posts:2010
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| 23 Aug 2010 05:22 PM |
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Who said ModCon? hehehee
I was thinking tank type. All the hype about standby losses won't add up either. If it is ROI we are focused on, the conventional water heater costs less and operating cost, maintenance, short zones, etc. are not a problem either. |
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Dana1
Senior Member
Posts:6991
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| 24 Aug 2010 11:36 AM |
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Posted By BadgerBoilerMN on 23 Aug 2010 05:22 PM
Who said ModCon? hehehee
I was thinking tank type. All the hype about standby losses won't add up either. If it is ROI we are focused on, the conventional water heater costs less and operating cost, maintenance, short zones, etc. are not a problem either.
You didn't say mod-con, but you did say: "...without outdoor reset and 25%+ more fuel usage..." ...which doesn't exactly spell "tank type" to me!?! If anything the condensing Navien cc210 is going to use at least 10% LESS than a tank type system in this app, even if set up with a fixed output temp and a bang-bang control scheme. If he didn't already have both a Navien and an electric HW tank I might agree, from the total cost POV a tank-type combi like the power-drafted version of the B-W Defender might make more financial sense. Since he's already invested in the Navien the cheapest way forward is probably the track he's on, and the efficiency won't be terrible- it'll be pretty decent with slab radiation. The thermal mass of the concrete is sufficient buffer to keep it from shortcycling itself into an early grave or low efficiency despite the ridiculously low heat loads. By tweaking the output temps and Navien-loop flow rate it should be pretty easy to hit "close enough" to keep it all happy without a lot of careful calculation. It won't ever average 98% AFUE on the space heaing load, but tweaked in it'll be well north of 90%. The best you'd get out of a propane fired tank combi in space-heating mode would be ~80% (in your dreams) due to raw combustion-efficiency limitations. |
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BadgerBoilerMN
Veteran Member
Posts:2010
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| 24 Aug 2010 02:05 PM |
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On line design...
I think you were just talking about this in a recent blog. My point is, as always, that a little professional design goes a long way. You know I like to remind the potential victims of online design disease.
I find the combi II at 80% combustion efficiency according to my Bacharach combustion analyzer.
Still the tank-less craze is a bad fad, more so when misapplied to space heating by the hapless DIYer. It is simply above the skill-set of the average homeowner who resorts to the Internet after the fact.
Operating efficiency for small loads is enhanced by the high mass, low temperature tank water heater. I like to dress up my combi water heaters with outdoor reset, increasing and operating efficiency at once.
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| MA<br>www.badgerboilerservice.com |
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jonr
Senior Member
Posts:5341
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| 24 Aug 2010 03:30 PM |
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The tankless water heater "fad" has been in place for 50+ years outside of the US and isn't going away - because it has some advantages.
In a weekend cabin, do what is inexpensive and easy. |
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NRT.Rob
Veteran Member
Posts:1741
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| 24 Aug 2010 03:34 PM |
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not nearly as many as it purports to have, though. when you dig into design elegance, passive heat storage actually solves a lot of issues than on demand heating attempts to solve with "Brute force".
Insulate a tank heater better, and in any regular use situation you could easily have an economic and efficiency improvement over on demand technology. I will admit that such tank heaters are currently restricted only to huge output ranges and costs, which is a shame.
Modulating gas valves and super slick heat exchangers are great, but lifecycle costing sure eats up a lot of DHW savings. |
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Dana1
Senior Member
Posts:6991
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| 24 Aug 2010 03:49 PM |
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Posted By BadgerBoilerMN on 24 Aug 2010 02:05 PM
On line design...
I think you were just talking about this in a recent blog. My point is, as always, that a little professional design goes a long way. You know I like to remind the potential victims of online design disease.
I find the combi II at 80% combustion efficiency according to my Bacharach combustion analyzer.
Still the tank-less craze is a bad fad, more so when misapplied to space heating by the hapless DIYer. It is simply above the skill-set of the average homeowner who resorts to the Internet after the fact.
Operating efficiency for small loads is enhanced by the high mass, low temperature tank water heater. I like to dress up my combi water heaters with outdoor reset, increasing and operating efficiency at once.
Raw combustion efficiency is the upper bound, not the AFUE. If it runs a 100% duty cycle, then that 80% will be the AFUE, but since it's more likely to be sub-5% average duty cycle, it'll be less than 80%. I suspect the raw combustion efficiency of the propane combi-II is a point or so higher than on the NG version, so maybe with the power-drafted version it'll make 80% AFUE. The standby losses that convect up & out the flue subract from that efficiency rating, and with something like a 95% standby time in this application it'll be more than just a percent or so. (Those losses are pretty small with the power-drafted version though.) There are no equivalent losses with a sealed combustion direct vented tankless. The high mass in this situation is the high thermal mass of the slab radiation itself- it needs no additional buffering to achieve maximal efficiency. In fact, the low return water temp from the slab allows the Navien to run well into the condensing zone 100% of the time, whereas the combustion efficiency of center-flue tanks is forever limited by being surrounded with water at or near the output temp, and by not being a counterflow heat exchanger. (Were it easy to make a tank centerflue HX into a counterflow arrangement with high turbulence on the water side it would be possible to get as high as 86% combustion efficiency out of , but it isn't. Condensing tank heaters have some rather unusual HX designs..) |
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ilgeo
Basic Member
Posts:180
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| 24 Aug 2010 04:45 PM |
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Some very nice advantages of a tankless for a vacation home are zero standby loss, no wait for a tank to reach set point, no tank full of stagnant water, and the fact that vacation homes tend to have periods of very high demand with high occupancy and long periods of no use. seems like an ideal application to me....Eric |
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BadgerBoilerMN
Veteran Member
Posts:2010
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| 24 Aug 2010 05:09 PM |
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Most are not using the condensing tankless and high mass means little.
When not heating domestic, design temperatures will be below body temperatures and standby losses irrevelant.
Having built condensing boilers and designed systems for same since the 80s, I am well aware "where" the mass is. I also know the difference between AFUE, combustion, and thermal efficiency.
But for the noise of a power vent, I would take it over the Navien.
Nice save, but the best design is the one made before you buy the goods. |
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| MA<br>www.badgerboilerservice.com |
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Dana1
Senior Member
Posts:6991
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| 24 Aug 2010 06:30 PM |
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Posted By BadgerBoilerMN on 24 Aug 2010 05:09 PM
Most are not using the condensing tankless and high mass means little.
When not heating domestic, design temperatures will be below body temperatures and standby losses irrevelant.
Having built condensing boilers and designed systems for same since the 80s, I am well aware "where" the mass is. I also know the difference between AFUE, combustion, and thermal efficiency.
But for the noise of a power vent, I would take it over the Navien.
Nice save, but the best design is the one made before you buy the goods.
Amen, brother Morgan! This place has two water heaters and two space heating systems already (if you count the wood burner) and he's making a third heating system out of one of the water heaters. But the latter will likely be his most-efficient option by a good margin. (It might be more expensive per BTU-delivered than the wood-burner though, depending on local fuel pricing.) The installed cost of all that (even if you don't include the wood burner) is likely comparable to or greater than a tiny mod-con + mid sized indirect that takes up even less space, and at least 2x that of a tank-combi. It wasn't my intent to lecture anyone on the differences in AFUE & combustion efficiency etc, only to reassert with prejudice that a tank-combi only hits 80% as a system in dreamland (and not even close, in this application), despite a measured combustion efficiency of 80% (or even 82%, which the propane version might hit.) Your quoted CE measurement datapoint on the combi-II seemed targeted at my prior assertion, but maybe I read too much into that. To be sure, on this (and most things) we agree completely: If starting on day-1 a tank combi makes the most sense from purely economic point of view, and would take only the amount of space that that beautiful electric tank/bypass-plumbing sculpture is now holding. In an intermittent use app like this the cost-delta between the condensing Navien and a combi-II never goes NPV+ on fuel savings within the lifetime of the equipment. It might eventually pay if propane prices quintuple over the next decade though, or if the place gets heated fully for the 9-10 months of heating season they typically see there. It may not be that cold in winter, but it's also not that warm in spring/fall. Even JUNE was a bit raw this year. It's about 5500 heating degree-days on average, but it's a widely distributed 5500HDD. But if the standby temp of the house is set to 55F, it has effectively zero standby heat load from April through October, but you'd still need to fire up the heating systems when using it pretty much any weekend of the year. (July/August average temps are only in the low to mid 60s there, so if un-occupied during the week it would likely be on the cool side when you first walked in for the weekend.) |
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heath
New Member
Posts:15
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| 24 Aug 2010 11:29 PM |
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These are all great points but my costs to get to where I am now are cheap pickked up the navien for 800 dollars maybe 200 in brass fittings used the same electric water heater that was there oh and the tube was 300$ a thousand so far around 1300 and a lot of my labor. If it was to easy it would take the fun out of it I like the challange. Looked at someboilers like the munchkin but seemed a lot more expensive and found a deal on the navien. The navien is virtually silent but wish I had the cc240 for more domestic flow. I think I am probably half way there to warm floors this winter. I can't believe how warm it stays down in the basement with no heat on in the dead of winter it feels warm down there compared to the other floors the electic fan heater 4 @ 6000 watts total take it to 70 quickly. You guys are educating me though I guess there are several ways to skin a cat you can never have to many sources of heat.
Thanks Heath |
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BadgerBoilerMN
Veteran Member
Posts:2010
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| 25 Aug 2010 10:07 AM |
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OK then, heheheee
and good luck to Heath, you have to love that Yankee engin u ity.
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Dana1
Senior Member
Posts:6991
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| 25 Aug 2010 10:32 AM |
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If you can do the math, look up specs and measure temp & flow, you can make this work, and it won't suck. Yours is a very different situation than the one MA was misconstruing it to be. I suspect in his minds-eye yours was akin to those cheap "one size fits all" radiant slab systems based around ~80-82% efficiency Takagis that are being hawked all over the internet. Those are a total waste when used a slab, since it's throwing away 20-25% of the total performance you'd get out of condensing burners.) Personally I'd never buy a canned system like that- I like doing the math and measuring stuff. If I didn't design it myself I'd hire a competent designer. (Even when I DO design it myself, I run the design, including the math that goes with it by a pro.) YMMV.
I'm a little surprised that you're not getting enough domestic hot water out of a 175K Navien though- with your ground temps I'd think the incoming cold water is warm enough that you could run 2 showers and a dishwasher simultaneously with it most of the year. I presume your water pressure meets spec, and the propane plumbing between the regulator and the unit are properly sized? (And the regulator can comfortably handle the full 175K with margin?) If the water pressure is lower than spec it'll cut into the efficiency of the heat exchange too, since at low pressure the size of the micro-bubbles on the water side of the unit's heat exchanger get big enough to be thermally insulating. (Most tankless units spec 15psi as the lower limit, but 25 or 30psi is safer bet.)
Installing the tankless correctly is the part where DIYers are more likely to run into trouble (sometimes disasterous trouble.) Undersized fuel lines, fuel lines shared with other significant loads, and undersized regulators etc. are common. Getting 25-50K out of a Navien and into the slab is relatively easy, but it pays to read up on hydronic design. There's more to it than a few rough ideas and some plumbing skills, but it's not rocket-science either. |
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heath
New Member
Posts:15
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| 25 Aug 2010 05:15 PM |
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Dana I guess I was not specific on why I wish I had used the 240 the 210 is great but my well pump pressure switch is set to 50-70 plenty of pressure and flow but If all fixtures are running I can probably dump 10 gal a min in hot water this is rare but it could happen in the summer with the outdoor shower in use and the dual head shower probably dumps 7gal min full tilt add laundry or dishwasher and maybe lose some pressure the navien will choke down the flow to maintain correct heat output. It is better though when I set navien to 105 and let the electric take it to 125 allows more flow through the navien. My ground water temp is around 55 and I believe the navien is firing completely ran 1 inch trac pipe off of 1 1/4 should give me plenty. It works great just wish I had that extra 3/4 or gallon of hot for the one time each year I need it. I tend to pack the place often in the summer since it is on the water. Thanks |
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Dana1
Senior Member
Posts:6991
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| 25 Aug 2010 06:07 PM |
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Posted By heath on 25 Aug 2010 05:15 PM
Dana I guess I was not specific on why I wish I had used the 240 the 210 is great but my well pump pressure switch is set to 50-70 plenty of pressure and flow but If all fixtures are running I can probably dump 10 gal a min in hot water this is rare but it could happen in the summer with the outdoor shower in use and the dual head shower probably dumps 7gal min full tilt add laundry or dishwasher and maybe lose some pressure the navien will choke down the flow to maintain correct heat output. It is better though when I set navien to 105 and let the electric take it to 125 allows more flow through the navien. My ground water temp is around 55 and I believe the navien is firing completely ran 1 inch trac pipe off of 1 1/4 should give me plenty. It works great just wish I had that extra 3/4 or gallon of hot for the one time each year I need it. I tend to pack the place often in the summer since it is on the water. Thanks
Since you're into spending the money on performance and extra hardware, even if there isn't much of a return on investment, and like plumbing projects...  If you have at least 4" of vertical drain downstream of your shower you can get quite a bit of performance out of drainwater heat recovery heat exchanger. Performance of theses beasties increases with both length and drain diameter, but you'll get better than 50% of the heat returned out of a 4" x 48" version. They're not super cheap though. Best deal you'll probably find on 'em state-side is by opening a commercial account with EFI (easy to do over the phone with a credit card in hand), the US distributor of the PowerPipe. They can be had off-the-shelf in many Home Despot stores in Canada (where they are subsidized), but at full-retail they're pricier still. (EFI will ship onesie-twosie on these, and don't mark up their shipping.) Natural Resources Canada pays for third-party testing of these beasties to a standardized spec, and maintains a relative-performance list by manufacturer & model here. Best performance is had when the drainwater HX feeds both the HW heater and the cold-feed to the shower (or the rest of the house.) Summertime output from the HX will be north of 80F, which means you may want to tap off to the kitchen ahead of it rather than running tepid- water on the cold taps everywhere whenever anyone is showering. The performance is real, and they have a ~40-50 year lifecycle before performance falls below 75% of brand-new performance. Being a counterflow heat exchanger, it only works for simultaneous flows (like showers), it won't make a soaking-tub fill any faster or return any of that energy when you pull the plug but it'll double your showering capacity, and cut the fuel use for showering in half (it really will!). I installed a 48-incher along with the tankless combi system I hacked into my own house in order to be able to run all the heating zones & two showers flat-out off a cheap 175KBTU Takagi. The system delivers the whole shebang mid-winter with burner to spare, but it would have been marginal at full on the hot water end without the drainwater heat exchanger. |
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